What Is AWG in Wiring? The Complete NEC Ampacity Chart

AWG stands for American Wire Gauge, the standardized US system for measuring the physical diameter of electrically conducting wire. In this system, a smaller AWG number indicates a thicker wire with lower electrical resistance and a higher current-carrying capacity (ampacity). For example, 10 AWG wire is significantly thicker and can safely carry more current than 14 AWG wire. Knowing what AWG in wiring means is the first step to sizing branch circuits, feeders, and overcurrent protection without violating the National Electrical Code (NEC) or creating a fire hazard.

The NEC 310.16 Ampacity Chart & Quick Reference

How to read this table: The table below is derived directly from NFPA 70 (NEC) Table 310.16. It assumes an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. The columns are split by conductor material (Copper vs. Aluminum) and insulation temperature rating (60°C, 75°C, and 90°C). Always read down the column that matches the lowest temperature rating of your wire, terminals, or breaker.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior electricians make is blindly using the 90°C column because modern THHN/THWN-2 wire is rated for 90°C. You cannot do this. The NEC enforces a weakest-link rule: your final allowable ampacity is limited by the lowest temperature rating of any connected component, terminal, or conductor in the circuit.

Here is how to select the correct column based on real-world installation types:

• The 60°C Column: Use this for standard nonmetallic-sheathed cable (NM-B, commonly known as Romex). Even though the individual THHN conductors inside NM-B are 90°C rated, NEC 334.80 strictly limits the ampacity of NM-B to the 60°C column. You must also use this column if you are connecting to older devices, receptacles, or breakers that are not explicitly marked with a 75°C rating.
• The 75°C Column: Use this for individual THHN/THWN-2 wires pulled through conduit (EMT, PVC) when terminating on modern, 75°C-rated breakers and lugs. Most standard residential breakers and subpanel lugs manufactured after the mid-1990s are rated 75°C.
• The 90°C Column: You are almost never allowed to use the 90°C column for your final breaker sizing. The 90°C column is only used as a starting point for derating calculations (explained below) before you apply the terminal temperature limits.

How Derating Modifies Your Base Ampacity

The ampacities in the table above assume ideal conditions: an ambient air temperature of 30°C (86°F) and no more than three current-carrying conductors (CCCs) bundled together. When conditions change, heat dissipation drops, and you must reduce (derate) the wire's capacity to prevent the insulation from melting.

1. Bundling Derating (NEC 310.15(C)(1))
When you pull more than three CCCs through a single conduit, the wires heat each other up. You must multiply the base ampacity by a correction factor. Note: You always use the 90°C column's base value for this math, then check if the result still satisfies your terminal limits.

• 4 to 6 CCCs: Multiply base ampacity by 80%.
• 7 to 9 CCCs: Multiply base ampacity by 70%.

Worked Example: You are pulling four 120V circuits through a single 3/4-inch EMT conduit. That equals 8 current-carrying conductors (four hots, four neutrals; grounds do not count). You are using 10 AWG THHN copper.
Step 1: The 90°C base ampacity for 10 AWG is 40A.
Step 2: 8 CCCs requires a 70% multiplier. 40A × 0.70 = 28A.
Step 3: Your derated ampacity is 28A. Because 28A is below the 30A limit of NEC 240.4(D) for 10 AWG, you can still safely protect this circuit with a 30A breaker. If you had used 12 AWG wire (90°C base 30A × 0.70 = 21A), you would be forced to downsize your breaker to 20A.

2. Ambient Temperature Derating (NEC 310.15(B)(1))
If your conduit runs through an attic in Texas where temperatures hit 110°F (43°C), you must apply an ambient temperature correction factor. For 90°C wire at 41-45°C, the multiplier is 0.82. You multiply the base 90°C ampacity by 0.82, and then apply the bundling multiplier if both conditions exist simultaneously.

What the AWG Table Cannot Tell You

While Table 310.16 dictates thermal limits (preventing the wire from catching fire), it completely ignores electrical efficiency and physical installation limits. Relying solely on the ampacity chart will lead to failures in three specific scenarios:

1. Voltage Drop Over Distance
A 12 AWG wire is perfectly legal on a 20A breaker for a 15-amp load. But if that receptacle is 150 feet away from the panel, the resistance of the wire will cause the voltage to sag. The NEC recommends a maximum 3% voltage drop on branch circuits. A 12 AWG copper wire carrying 15A on a 150-foot one-way run will drop roughly 9.5V (almost 8%). Your tools and electronics will underperform, and motors may burn out. To fix this, you must upsize to 8 AWG or 6 AWG wire purely for voltage drop management, even though the ampacity table says 12 AWG is thermally safe.

2. Conduit Fill Capacity (NEC Chapter 9)
The ampacity table doesn't tell you if the wires will physically fit in the pipe. If you try to pull four 6 AWG THHN wires into a 1/2-inch EMT conduit, they will jam, and you will strip the insulation off the wires during the pull. You must cross-reference NEC Chapter 9, Table 1 to ensure your conduit cross-sectional area is not exceeded (typically 40% fill for three or more wires).

3. Physical Pulling Tension
For large feeders (like 2/0 or 4/0 AWG aluminum for a 200A service upgrade), the physical weight and friction of the cable can exceed the pulling tension limits of the conductor, stretching the metal and compromising the termination. This requires calculating pulling tension and using specialized wire-pulling lubricants, data which exists entirely outside the AWG ampacity charts.